Within the drilling industry, LWD (Logging While Drilling) devices have used acoustic pulse-echo measurements to measure a variety of parameters of a bore hole such as standoff, caliper, imaging and the like. These devices have one or more acoustic transducers that emit an acoustic pulse toward a surface being probed and then receive a reflected signal. Time between when the acoustic pulse is emitted and when a reflected signal is received (acoustic transit time) can then be used in the determination of distance. When the velocity of sound in a medium is known, acoustic transit time can be used to determine exact distance between the acoustic transducer and the surface being probed. Because the acoustic transducer of these devices is positioned such that at least some if not all of the acoustic signal path propagates through a fluid medium in the bore hole (e.g. a liquid such as drilling mud or mud), inaccuracies are introduced into the value of the speed of sound in the fluid medium (known as the acoustic velocity of the medium). Inaccuracy in the measurement of acoustic velocity translates into inaccuracy in the determination of distance. In the context of a bore hole, presence of the drilling mud creates inaccuracies into the determination of distance given that the precise acoustic velocity of the mud is not known.
Because drilling mud is formulated to exhibit particular properties, each formulation has a unique acoustic velocity. In addition, during the drilling process the speed of sound in the mud is determined by factors other than its initial unique acoustic velocity such as mud type (oil or water), mud weight, mud density, mud temperature, mud pressure, the amount of cuttings in the mud, the amount of formation fluids entering the mud, and the like. Conditions change such that the acoustic velocity in the mud changes. Therefore, it is difficult to accurately determine acoustic velocity in a fluid medium within a bore hole during drilling.
Because many LWD bore hole measurements use acoustic velocity to determine distance, it is desirable during the drilling process to be able to accurately determine the acoustic velocity in a medium in a bore hole. Moreover, it is desirable to be able to accurately calibrate the various LWD measurements through accurate measurement of acoustic velocity in a medium in a bore hole during the drilling process. Moreover, it may be desirable to determine mud velocity or speed within the bore hole.
The problems in the prior art, the desirables presented above and more are addressed by the present invention.